#include<bits/stdc++.h>
using namespace std;
#define foru(i,a,b) for(int i=(a); i<=(b); ++i)
#define ford(i,a,b) for(int i=(a); i>=(b); --i)
#define rep(i,a) for(int i=0; i<(a); ++i)
#define sz(a) (int)(a).size()
#define all(a) (a).begin(),(a).end()
#define bit(s,i) (((s)>>(i))&1)
#define ii pair<int,int>
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
#define ll long long
#define _ << " " <<
template <class X, class Y> bool maxi(X &x, Y y){return x<y?x=y,true:false;}
template <class X, class Y> bool mini(X &x, Y y){return x>y?x=y,true:false;}
const int N=404;
int n,a[N],sum[3][N],pos[3][N],cpos[3];
string s;
int get(char c){
if(c=='R')return 0;
if(c=='G')return 1;
if(c=='Y')return 2;
return -1;
}
void solve(){
cin>>n;
cin>>s;
rep(i,n) a[i+1]=get(s[i]);
// foru(i,1,n)cout<<a[i]<<" ";
// cout<<'\n';
rep(i,3){
foru(j,1,n){
sum[i][j]=sum[i][j-1]+(a[j]==i);
if(a[j]==i){
pos[i][++cpos[i]]=j;
}
}
}
ll dp[cpos[0]+1][cpos[1]+1][cpos[2]+1][3];
memset(dp,0x3f,sizeof(dp));
dp[0][0][0][0]=0;
foru(i,0,cpos[0]) foru(j,0,cpos[1]) foru(k,0,cpos[2]) foru(lst,0,2){
if(i<cpos[0] && (i+j+k==0 || lst!=0)){
int p=pos[0][i+1];
mini(dp[i+1][j][k][0], dp[i][j][k][lst] +
p + max(0,i-sum[0][p]) + max(0,j-sum[1][p]) + max(0,k-sum[2][p]) - (i+j+k+1));
}
if(j<cpos[1] && (i+j+k==0 || lst!=1)){
int p=pos[1][j+1];
mini(dp[i][j+1][k][1], dp[i][j][k][lst] +
p + max(0,i-sum[0][p]) + max(0,j-sum[1][p]) + max(0,k-sum[2][p]) - (i+j+k+1));
}
if(k<cpos[2] && (i+j+k==0 || lst!=2)){
int p=pos[2][k+1];
mini(dp[i][j][k+1][2], dp[i][j][k][lst] +
p + max(0,i-sum[0][p]) + max(0,j-sum[1][p]) + max(0,k-sum[2][p]) - (i+j+k+1));
}
}
ll res=1e18;
rep(i,3) mini(res, dp[cpos[0]][cpos[1]][cpos[2]][i]);//, cout<<i _ res<<'\n';
if(res>1LL*n*(n-1)/2) cout<<"-1\n";
else cout<<res<<'\n';
}
int32_t main(){
#define task "test"
if(fopen(task".inp","r")){
freopen(task".inp","r",stdin);
freopen(task".out","w",stdout);
}
cin.tie(0)->sync_with_stdio(0);
int tc=1; //cin>>tc;
rep(i,tc){
solve();
}
}
